1. Field of the Invention
The present invention is directed towards polysaccharides modified with various moieties, including carboxylate, aldehyde, sulfonate, phosphate and phosphonate moieties. The present invention is also directed towards anti-scalant and/or dispersant formulations or compositions including such polymers and their use in aqueous systems, including scale minimization.
2. Background Information
Aqueous systems, such as boiler water or steam generating systems, cooling water systems, gas scrubbing systems, pulp and paper mill systems, desalination systems, fabric, dishware and hard surface cleaning systems and downhole systems encountered during the production of gas, oil, and geothermal wells, are well known. Often the water in these systems, either naturally or by contamination, contains ingredients such as inorganic particulates and/or salts. Inorganic particulates such as mud, silt and clay, tend to settle onto surfaces, thereby restricting water flow and heat transfer unless they are effectively dispersed. The salts can cause accumulation, deposition, and fouling problems in these aqueous systems if they are not kept in a soluble, suspended or dispersed state.
The inorganic salts are typically formed by the reaction of metal cations such as calcium, magnesium or barium with inorganic anions such as phosphate, carbonate and sulfate. When formed, the salts tend to be insoluble or have low solubility in water. As their concentration in solution increases, or as the pH and/or temperature of the water containing those salts changes, the salts can precipitate from solution, crystallize and form hard deposits or scale on surfaces. Such scale formation can be problematic in equipment such as heat transfer devices, boilers, secondary oil recovery wells, and automatic dishwashers, as well as on substrates washed with such hard waters, reducing the performance and life of the equipment.
In addition to scale formation, many cooling water systems made from carbon steel such as industrial cooling towers and heat exchangers experience corrosion problems. Attempts to prevent this corrosion are often made by adding various inhibitors such as orthophosphate and/or zinc compounds to the water. However, phosphate addition increases the formation of highly insoluble phosphate salts such as calcium phosphate. The addition of zinc compounds can lead to precipitation of insoluble salts such as zinc hydroxide and zinc phosphate.
Stabilization of aqueous systems containing scale-forming salts and inorganic particulates involves a variety of mechanisms. Inhibition is the conventional mechanism for eliminating the deleterious effect of scale-forming salts. In inhibition, one or more polymers are added that increase the solubility of the scale-forming salt in the aqueous system.
In treating cooling water, phosphonates and low molecular weight homopolymers tend to be the primary calcium carbonate inhibitors. However, these additives may not be enough under stressed conditions. Therefore there is a need for a polymer that can act as a crystal growth modifier for crystals formed in stressed conditions. Inhibitors previously mentioned may not be completely effective.
Another stabilization mechanism is the dispersion of precipitated salt crystals. Synthetic polymers having carboxylic acid groups function as good dispersants for precipitated salts such as calcium carbonates. In this mechanism, the crystals stay dispersed rather than dissolving in the aqueous solution.
A third stabilization mechanism involves interference and distortion of the crystal structure of the scale by the polymer, thereby making the scale less adherent to surfaces, other forming crystals or existing particulates.
The addition of synthetic polymers to cleaning compositions can also impart many useful functions to those compositions. For example, they can function either independently or concurrently as viscosity reducers in processing powdered detergents. They can also serve as anti-redeposition agents, dispersants, scale and deposit inhibitors, crystal modifiers, and/or detergent assistants capable of partially or completely replacing materials used as builders while imparting optimum detergent action properties to surfactants.
Cleaning formulations typically contain builders such as phosphates and carbonates for boosting their cleaning performance. However, these builders can also precipitate out insoluble salts such as calcium carbonate and calcium phosphate (in the form of calcium orthophosphate). The salt precipitants form deposits on clothes and dishware, resulting in unsightly films and spots on these articles. Similarly, insoluble salts can cause major problem in downhole oil field applications. Hence, there remains a need for polymers that minimize scaling from insoluble salts found in water treatment, oil field and cleaning formulations.
Synthetic polymers have been used to minimize scale formation in aqueous treatment systems for a number of years. For example, polymers such as polyacrylic acid have been used as calcium binding or calcium carbonate inhibiting agents, co-builders and dispersants in detergent and water treatment applications for decades. However, due to rising demand and tight crude oil supplies, there has recently been a shortage of monomers used in producing these synthetic polymers, driving up production costs for the polymers. Hence, there is a need for polymers from renewable natural sources to replace these synthetic polymers. As an added benefit, polymers from renewable natural sources should have a better biodegradability profile than synthetic polymers, which tend to have very little biodegradability.
Starches, including oxidized starches as well as other modified polysaccharides, have shown potential use as detergent additives in the past. For example, it is known to use oxidized inulin as calcium binding agents. Also, polysaccharide co-builders include those produced from oxidized glucosan, oxidized dextrins for use in detergent formulations, and low molecular weight carboxylated maltodextrins, likewise for use in detergent applications. Further, it is known to use modified polysaccharides as chelating agents. However, their performance has been weak at best when compared to synthetic polymers like polyacrylic acid.
Synthetic polymers are typically produced from petroleum-based feedstocks. The price of raw materials used to produce such synthetic polymers has risen sharply in the past few years. Accordingly, modified polysaccharides according to the present invention offer a benefit over synthetic polymers on a cost-performance basis.
It is also recognized that both synthetic and natural polymers containing carboxylic acid groups function well at inhibiting calcium carbonate. Generally speaking, the greater the amount of carboxylate functionality, the greater the amount of calcium carbonate inhibition that is provided. However, in most environments the amount of scale present is usually greater than what the polymer can inhibit. Hence, there is a need for polymers that can minimize calcium carbonate scale by a dispersion or crystal growth modification.
Polysaccharides modified with carboxylated groups have been previously proposed for calcium binding and hardness control. However, these naturally derived products tend not to be as effective as synthetic polymers in such applications, which is due at least in part to the limited amount of carboxylic groups that can be introduced onto the polysaccharide backbone. Therefore, synthetic polymers such as polyacrylic acid tend to contain more carboxylic acid functionality per gram of material than even highly oxidized polysaccharides. Furthermore, as the degree of substitution or oxidation increases, the biodegradability of polysaccharides decreases. Therefore, there still is a need for polysaccharides capable of functioning as scale control agents, dispersants and/or soil suspension agents.
Blends of synthetic polymers and natural polymers tend to be incompatible. Therefore, there is also a need for synthetic and natural polymers that are compatible with one another, enabling them to be blended together.